Immunoglobulin A (IgA) nephropathy is the most common type of primary glomerulonephritis in the world. A wealth of literature suggests that vitamin D and its analogs have profound effects on immune system function and glomerular mesangial cell proliferation. Calcitriol, an active form of vitamin D, is commonly used for the treatment of secondary hyperparathyroidism in patients with advanced chronic kidney diseases. Therefore, the investigators plan to conduct a open-label single-arm study to evaluate the safety and efficacy of calcitriol in the treatment of IgA nephropathy. Ten patients with biopsy-proven IgA nephropathy and persistent proteinuria despite conventional therapy will be recruited. They will be treated with calcitriol for 12 weeks. Proteinuria, renal function, serum and urinary inflammatory markers will be monitored. This study will explore the potential anti-proteinuric and anti-inflammatory effects of calcitriol in the treatment of IgA nephropathy, which is a major cause of dialysis-dependent renal failure.